The subjects in this study were drawn from the Vietnam Head Injury Study registry, 92% of whom had a history of a penetrating head injury. Phase 3 (P3) has been modeled upon the Phase 2 (P2) Vietnam Head Injury Study. Of the 520 head-injured subjects that were assessed in P2, 484 are still alive, and 200 attended P3 of the study. Additionally, 20 patients identified in Phase 1 but that did not attend P2 were assessed. Of the original 80 control subjects without head injuries recruited in P2, 32 attended P3 and a further 27 were recruited for P3, through advertisements in veteran publications. Subjects were assessed over 5-7 days at the National Naval Medical Center in Bethesda, Maryland.[unreadable] There were no significant differences between the head-injured and control subjects in terms of age at P3 testing, years of education or induction intelligence level (as measured by the Armed Forces Qualification Test; AFQT)[unreadable] One genetic marker that was found to significantly predict overall change in AFQT score from P2 to P3 was GRIN2A: 8212 (F=4.033; p=0.020, df=2). All subjects with GRIN2A: 8212 were either heterozygous dominant (A1), heterozygous recessive (A20) or were homozygotes. An ANOVA procedure was carried out to see if each of these groups had an increased significance of decline compared to those without a GRIN2A: 8212 allele. Whilst there was a trend for dominant heterozygotes to have a greater level of decline in intelligence from P2 to P3 (mean=-11.714, sd=15.663) compared with recessive heterozygotes (mean=-9.071, sd=12.608) and homozygotes (mean=-8.519, sd=10.988), this did not reach significance. [unreadable] Again, the only genetic marker that was found to significantly predict overall change in AFQT score from preinjury to P3 was GRIN2A: 8212 (F=3.802; p=0.025, df=2). An ANOVA procedure was carried out to see if the those with a GRIN2A: 8212 allele showed a increased level of decline in intelligence from preinjury to P3. Again although there was a trend for dominant homozygotes to have a greater level of decline in intelligence from P2 to P3 (mean=-9.429, sd=22.849) compared with recessive homozygotes (mean=-8.021, sd=18.432) and heterozgotes (mean=-5.548, sd=18.205), this did not reach significance.